With a conventional threaded fastener, for example, in the form of a nut, a significant proportion of the energy required to tighten the nut on a threaded spindle of a bolt or the like, is dissipated due to friction between the nut and the surface of an object to be fastened. This means that the torque required to achieve the necessary "tightness" of the nut, (i.e. the hold-down pressure exerted by the nut on the object to be fastened), can be up to fifty percent greater than would otherwise be necessary. For this reason, special tools such as, for example, a long spanner or an adjustable torque wrench, are used to provide the torque necessary to overcome the friction and still achieve the desired hold-down-pressure. For the same reason a special tool is usually required to unfasten the nut as well.
It was hitherto thought that maximum friction is required between the nut and the fastened object in order to ensure that the nut and/or the object did not come unfastened in use. A recent discovery suggests that this may not be the case, and that provided sufficient hold-down pressure is exerted by the action of the nut against the fastened object, the nut will remain fastened due to the reactive tensioning thereof and the frictional grip on the thread of the spindle on which the nut is threaded. In many situations, the object to be fastened cannot rotate relative to the spindle, for example, because it is keyed to the spindle, so that the possibility of the nut unfastening by itself is significantly reduced anyway. However, the discovery that little or low friction is required between the nut and the fastened object to keep the nut fastened, makes it desirable to minimize such friction so that more of the torque is available to be converted to hold-down pressure.
U.S. Pat. No. 4,969,788 discloses a nut with a pressure ring of the type having a spherically shaped undersurface for engagement with a complementary shaped depression in the component to be fastened. The invention seeks to overcome problems associated with conventional slit pressure rings which are normally a separate component from the nut. A nut with a closed pressure ring connected thereto is provided, the means of connection permitting the nut and ring to rotate relative to one another about the nut axis and to shift relative to one another laterally of the nut axis. The nut and the pressure ring have confronting pressure transmitting surfaces, and at least one of the pressure transmitting surfaces is provided with a plurality of recesses for holding lubricant. This can be achieved for example by shot peening the pressure transmitting surface. According to U.S. Pat. No. 4,969,788, in this manner the shot peened pressure transmitting surface can be provided with a permanent lubricating film whereby friction between the nut and the pressure ring is not of the same magnitude as with a slit pressure ring and a debasing of the hold-down-torque during tightening of the bolt is reduced. The opposite confronting pressure transmitting surface is preferably finely turned or polished.
Presumably the lubricant employed is a liquid or viscous lubricant, for example grease, so that if the recesses in the shot peened surface are filled with the lubricant, as the nut and pressure ring are rotated relative to one another some of the lubricant in the recesses sticks to the confronting surface and is spread as a thin film between the pressure transmitting surfaces in known manner. This may prove satisfactory for a time, however after repeated use many of the recesses will be evacuated of lubricant and some of the lubricant will have been squeezed out from between the confronting surfaces so that little or no lubricant is left to form the thin film therebetween. Furthermore, liquid and viscous lubricants are prone to dry out and solidify over time. Accordingly, the hold-down-torque of the nut and pressure ring of U.S. Pat. No. 4,969,788 will be progressively debased during repeated tightening and loosening. More importantly, the liquid or viscous lubricant is unable to hold the two confronting surfaces fully apart so that there will always be a degree of metal to metal contact with the attendant friction therebetween. This problem is only exacerbated as the amount of lubricant between the pressure transmitting surfaces decreases over time.
The present invention was developed with a view to providing an improved fastener in which an increased proportion of the torque required to fasten an object can be converted to pressure applied to the object, even during, and especially for, repeated tightening and loosening. Advantageously, the improved fastener can also be readily fastened by hand since less torque is required to achieve the same hold-down pressure as a conventional fastener thus the need for a tool to fasten or unfasten the fastener may be eliminated. The remainder of the specification will be directed primarily to one application of the invention, namely fastening a rotary tool such as a grinding wheel, circular saw, carving disc or router blade to a power tool, however it will be appreciated that the invention has much wider application, for example, in the automotive industry.
A conventional clamp nut used for securing a grinding wheel to the spindle of an angle grinder is threaded through its central axis and cooperates with a washer-like backing plate which precedes the wheel onto the spindle. Shoulders provided on the spindle lock the backing plate to the spindle, and as the clamp nut is screwed onto the spindle it applies pressure to the wheel so that it is clamped against the backing plate and hence secured to the spindle. Conventionally, the friction grip of the plate on the grinding wheel when the nut is tightened prevents the wheel from turning on the spindle. A spanner or similar tool may be used to tighten the nut securely. Moreover, the clamp nut will also tighten spontaneously during use as the grinding wheel counter rotates relative to the direction of rotation of the spindle, when the wheel engages the work piece, so much so that it can be extremely difficult to remove the nut after use. Several improved types of fastener are known which are intended to eliminate the need for a special tool to loosen the fastener on the spindle and to make it easier to replace the grinder wheel or carving disc.
For example, AU 12247/88 (594557) by ROBERT BOSCH GmbH discloses a clamping device for axial clamping of a disc onto a flange of a spindle of a power tool. The clamping device comprises a clamp nut which can be screwed onto the spindle of a power tool, such as a portable grinder, and which supports a non-rotatable and axially movable clamp disc. The clamp disc engages the grinding wheel and is adapted to be loaded by the clamp nut with an axially directed compressive force and thus to press axially against the grinder wheel. An operating member is disposed between the clamp nut and the turning movement of the operating member axially releases the clamp disc from a clamped condition. Thus, the clamping device of AU12247/88 (594557) can facilitate easy release of the clamp nut by hand even when the latter is locked tight on the tool spindle. However, in certain circumstances a special tool may still be required, for example, when the clamping device is in a rusted-on state or is used in the unlock position. Furthermore, the construction of the clamping device is extremely complicated with many moving parts and it is therefore expensive to manufacture. Moreover the operating member must protrude sufficiently outwards from the grinding wheel in order to be gripped by the operator's hand and therefore presents an inconvenient obstruction during use. Also this clamping device is prone to spontaneously overtighten with the turning of the grinding wheel and this is known to cause failure of the grinding wheel, apart from the usual problems of overtightening.